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The Unix Shell: Lesson 4. Pipes and Filters
Word count without flags is used to initially introduce the command and outputs three points of data - character, word, and line count. Then -l is added to show how line count is attained. It would be helpful to also include wc -cwl to show that the output is the same as default without flags. This would help by teaching about default flags and also increase exposure to varied usage by showing -c and -w in addition to -l. A possible addition (without showing the output):

$ cd molecules
$ wc *.pdb

The default flags for the wc command are -cwl. If we run wc -cwl instead of just wc, the output will be identical:

$ wc -cwl *.pdb

Each flag can also be used individually. By running wc -l instead of just wc, the output shows only the number of lines per file: